Long Term Evolution (LTE) is an improved universal mobile telecommunication system (UMTS) that provides higher data rate, lower latency and improved system capacity. In LTE systems, an evolved universal terrestrial radio access network includes a plurality of base stations, referred as evolved Node-Bs (eNBs), communicating with a plurality of mobile stations, referred as user equipment (UE). A UE may communicate with a base station or an eNB via the downlink and uplink. The downlink (DL) refers to the communication from the base station to the UE. The uplink (UL) refers to the communication from the UE to the base station. LTE is commonly marketed as 4G LTE, and the LTE standard is developed by 3GPP.
Spatial multiplexing is a transmission technique in multiple-input multiple-output (MIMO) wireless communication to transmit independent and separately encoded data signals called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed more than one time. If the transmitter is equipped with Nt antennas and the receiver has Nr antennas, the maximum spatial multiplexing order (the number of streams) is Ns=min (Nt, Nr) if a linear receiver is used. This means that Ns streams can be transmitted in parallel, ideally leading to an Ns increase of the spectral efficiency.
In a semi-open-loop MIMO system with Nt transmitter antennas and Nr receiver antennas, the input-output relationship can be described as yi=HiWixi+ni, where i is the resource element (RE) index, yi, xi, ni are the (Nt×1) vectors of the received symbols, the transmitted symbols, and noise, Hi is the (Nr×Nt) matrix of channel coefficients, and Wi is the (Nt×Ns) linear precoding matrix on the i-th RE. A precoding matrix is used to precode the symbols to enhance the performance. When the precoding matrix on the reference signal RE is the same as that on the data RE in a resource block, the precoding matrix is transparent to the UE and therefore the UE doesn't need to know W. On the other hand, the data RE can be assigned the precoding matrix that is different from the one on the reference signal in a resource block. For example, in a resource block, the precoding matrix on the reference signal is W, and the precoding matrix on the data RE is WUi. Then Ui should be specified for each RE index and is non-transparent to the UE.
In LTE systems, a two-stage codebook has been defined for the precoding matrix. The precoding matrix to be applied to the data RE is represented by equation (1) below, and the precoding matrix on the reference signal (RS) which is transparent to the UE becomes equation (2), and equation (3) is the co-phasing cycling matrix for the data RE.
                                          1                                          2                ⁢                P                                              ⁡                      [                                                                                                      v                      _                                                              l                      ,                      m                                                                                                            0                    _                                                                                                                    0                    _                                                                                                              v                      _                                                              l                      ,                      m                                                                                            ]                          ⁡                  [                                                    1                                            1                                                                                      φ                  n                                                                              -                                      φ                    n                                                                                ]                                    (        1        )                                          1                                    2              ⁢              P                                      ⁡                  [                                                                                          v                    _                                                        l                    ,                    m                                                                                                0                  _                                                                                                      0                  _                                                                                                  v                    _                                                        l                    ,                    m                                                                                ]                                    (        2        )                                [                                            1                                      1                                                                          φ                n                                                                    -                                  φ                  n                                                                    ]                            (        3        )            
LTE has defined different transmission mode (TM) for different transmission scenarios. For transmission mode 9 or 10, up to 8 layers transmission is supported with UE-specific reference signal, e.g., demodulation reference signal (DMRS). For DMRS-based TM, the UE maps assume the ratio of physical downlink share channel (PDSCH) energy per resource element (EPRE) to UE-specific RS EPRE within each OFDM symbol containing UE-specific RS is 0 dB for the number of transmission layers less than or equal to two. However, based on the precoding matrix on the data RE (equation (1)), and the precoding matrix on the UE-specific RS (equation (2)), the PDSCH EPRE to UE-specific RS EPRE ratio is 3 dB, which violates the specification of 0 dB. Also, when the DMRS power is lower, it will impact the channel estimation performance at the receiver.
A solution is sought.